Synthesis and characterization of commercial pure titanium-nickel alloy behavior reinforced with titanium diboride

Commercial pure titanium alloy with Ni-TiB 2 ceramic additions (5, 10, 15 and 20 vol.%) were synthesized through the spark plasma sintering approach with sintering temperature of 1000 o C, the heating rate of 100 o C/min, holding time of 5 min at a constant pressure of 50 MPa. The study investigated the effect of Ni-TiB 2 on the densification, phase change, microhardness, microstructure, and wear properties of the sintered titanium-based composites. Results showed that Ti-Ni-TiB 2 composites relative density ranges from 97 to 99 %, while microhardness values increase with addition of nickel and titanium diboride from 228 to 587 HV 0.1 . The microstructural evolution shows that pure titanium transformed from lamellar phase to equiaxed alpha phase upon addition of nickel alloy and further get refined with a distinct grain boundary comprises of titanium diboride around the boundaries. The average coefficient of friction for the titanium-based composite was higher for commercially pure titanium (0.73) while the addition of TiB 2 exhibit (0.66, 0.63, 0.58, 0.55 and 0.46 respectively) improvement in the wear behavior.


Introduction
Titanium and titanium alloys have been utilized in different engineering applications because of their attractive properties, for example, low density, high specific strength, biocompatibility and exceptional corrosion resistance [1,2]. Despite the properties that titanium alloys with usage in various engineering applications, they are restrained for high-temperature applications usage due to their poor tribological properties. The way toward improving the mechanical properties of titanium alloys can be accomplished through the addition of various alloying elements, for example, alpha stabilizer, neutral, beta isomorph and eutectoid stabilizers. Nickel is beta eutectoid stabilizers which at high temperature, nickel and titanium alloy form intermetallic compounds which is ductile with better plasticity, high impact resistance and good damping properties [3][4][5].
Titanium diboride (TiB2) have attracted increasing consideration from various researchers because of its various applications such as, cutting tools, wear applications, high-temperature structural materials and lightweight impact resistance protective layer material, because of its exceptional mechanical properties, for example, high melting point, high hardness, high Young's modulus, good abrasion resistance, high strength, thermal conductivity and chemical stability [6][7][8].
Metal matrix composites have shown remarkable potential for the structure and improvement of innovative new advanced materials which is achieved in consolidating ceramic as methods for strengthening metal matrix to improve its mechanical properties [9]. Metal and its alloy don't completely possess some degree of properties required in essential materials and this driven the improvement through metal matrix composites by reinforcing it either utilizing particulate or ceramic into the metal matrix [10,11].
Spark plasma sintering is another sintering method that permits sintering of powders at lower sintering temperatures, rapid heating, holding and cooling time when identified with different conventional sintering technique [12]. In SPS process, because of its highlights, such as, raw powders and graphite die are directly heated with a pulsed current which prompts the making of sparks between the powder particles as a result generate spark discharge and mass transfer which can be accomplished rapidly and further simplicity expulsion of oxide contaminants from the surface of the particles [13]. The application of pressure can deform the particles and accordingly permitting the production of stronger connections between neighboring particles whereby the 3 electrical field can help in atom diffusion [14]. Spark plasma sintering methods have demonstrated that rapid time and heating rates consolidation has been advantageous in preventing grain development and control of microstructures thereby retaining the grain size and achieving improvement in mechanical properties [15,16] with high relative densities in a short timeframe.
In this study, TiB2 particle-reinforced titanium-nickel based composites were successfully created utilizing a spark plasma sintering method. The effects of Ni with ceramic particle (TiB2) additions at the distinctive volume per cent composition on the densification, hardness, microstructures and wear behaviour were examined.  The wear test was completed utilizing a tribometer pin-on-disk friction module procedure. A stainless-steel ball with 6 mm in diameter was utilized as a counter face rubbing against the titanium-based composite. A load of 10 N was applied with a rotational speed of 300 rpm under a dry rotary condition at ambient temperature. The mean coefficient of friction (COF) was determined and reported for all materials. The volume loss (V) of the titanium-based composites was determined utilizing the below equation 1:

Experimental procedure
Where Δw= weight before test-weight after test and ρ = density of the titanium-based composites.
The wear rate (W) was then determined to utilize the below equation 2: Where Δw is weight loss difference (before-after tests), ρ is density of the titanium-based composites, L is sliding distance and F is the applied force. vol.% TiB2. The powders resulted in uniform relatively dispersion within the surface of the titanium alloy particles. Also, the shape of the titanium particles can be found to have change a little with some agglomerated particles which might be due to cold welding during the mixing operation. The process of powder blending approach is typically noticeable by the homogeneity of the blended powder [17]. Figure 1f Figure 4a shows the effect of sintering parameters on the densification behavior. The relative density decreased with the addition of reinforcement from 99 to 97 %. However, the incorporation of ceramic reinforcement into materials tends to decrease their density [21]. The decrease shows that some few porosities might be present in the composite material. Figure 4b shows the microhardness value which increases as the reinforcement is addition in different percentage. The increase in microhardness can be attributed to the high hardness characteristics of the additional ceramic additives which prevent dislocation movement and give a good resistance to plastic deformation of the titanium matrix composites [22]. Also, the increase in hardness of the titanium matrix composite can be attributed to the homogenous dispersion of the particles.  indices. Addition of nickel into titanium alloy causes a reduction in the intensities peak, with a slight shift in the peaks which, could be as a result of high tensile stress of α phase [23]. However, diffraction peaks of the sintered titanium-based composite confirm the presence of both α and β peak, which are predominantly phases [24]. On the other hand, an increase in the peak height with the addition of 5-20 vol.% TiB2 was observed. Generally, the decrease in the peak height of the titanium-based composite could likewise be credited to the x-ray absorption effect [24]. Thus, some peaks were observed which are different from the primary titanium alloy such as (106),  taking place at the composite and ball contact surfaces [25]. The wear volume and wear rate of the titanium-based composites are presented in Fig. 7b-c. It is seen that the wear volume is responsive to the extent of the applied load. It is noticed that the wear volume decreases with increase in the 11 reinforcement (TiB2) content, which can be attributed to the hardness of titanium-based composites which displayed an increase as the reinforcement contents are added into the titanium alloy. 2. Addition of alloying element and ceramic particles (Ni-TiB2) to the titanium matrix significantly influence through increasing the microhardness value and causing a transformation in the microstructure by hindering grain development and grain boundary movement. However, there was a decrease in the density of the produced titanium matrix composite with an increase in the concentration of reinforcement. 12 3. The microstructural evolution shows the transformation from lamellar into an equiaxed phase with titanium diboride situated at the grain boundaries of the titanium matrix composite. There is additionally a good interfacial bonding between the titanium matrix and both Ni-TiB2 support materials.

Density and hardness measurement of sintered titanium-nickel based composite
4. Titanium-nickel based composites exhibited good wear resistance when related to commercially pure titanium. Increase in the reinforcement addition displayed a lower coefficient of friction compared to titanium alloy.